1. Field of the Invention
The present invention relates to a reference current generator circuit of an organic EL drive circuit, an organic EL drive circuit using the same reference current generator circuit and an organic EL display device using the same organic EL drive circuit. Particularly, the present invention relates to a reference current generator circuit for a display device of an electronic device such as a portable telephone set or a PHS, which is suitable for use in high luminance color display and with which white balance of the display device is unchanged within a wide temperature range by preventing loss of white balance due to change of temperature environment of the display device.
2. Description of the Related Art
An organic EL display panel of an organic EL display device for use in a portable telephone set, a PHS, a DVD player, PDA (portable digital assistances), etc., including 396 (132×3) terminal pins for column lines and 162 terminal pins for row lines has been proposed. The number of the terminal pins for column lines and row lines is still increasing.
An output stage of a current drive circuit of a conventional organic EL display panel of either the active matrix type or the passive matrix type includes output circuits, which are provided correspondingly to respective terminal pins of the panel and each of which is constructed with a current source drive circuit constructed with such as, for example, a current mirror circuit.
Further, in order to keep white balance on a display screen of a color organic EL display panel unchanged by regulating luminance of R (red), G (green) and B (blue) primary colors on the screen correspondingly to organic EL materials for the primary colors, the current drive circuit of the color organic EL display panel includes regulation circuits for luminance regulation of the respective R, G and B colors on the screen.
Each of the current drive circuit for driving the organic EL elements for R, G and B colors arranged in a matrix usually includes a single reference current generator circuit commonly used for R, G and B colors and reference current setting circuits provided correspondingly to the respective R. G and B. The reference current setting circuits provided correspondingly to the respective primary colors regulate a reference current generated by the single reference current generator circuit to generate reference currents corresponding to the respective primary colors. White balance on the display screen is realized by regulating the reference current by the reference current setting circuits. The thus regulated reference currents for R, G and B colors are supplied to the drive circuits for the respective R, G and B colors.
JP2001-34221A discloses a technique in which an organic EL element emits light by supplying a constant current from a constant current setting circuit to a drive circuit of the organic EL element and driving the drive circuit with PWM pulse and in which reduction of intensity of emitted light due to aging is recovered by regulating the current from the constant current setting circuit.
In general, an electronic device, which has an organic EL display device, is operable in a wide temperature range (temperature environment) from −10° C. to +70° C. In such wide operable temperature range of the display device, loss of white balance on a display screen due to environmental temperature change of the electronic device in such temperature range becomes a problem in, particularly, a case where the organic EL display device is a high luminance color organic EL display device. Such loss of white balance on the display screen is caused by the fact that the light emitting characteristics of R, G and B colors are changed correspondingly to temperature and dependently upon luminous material forming the organic EL elements.
Presently, a temperature characteristics R of a luminous material for R color is linearly reducing with increase of temperature as shown by a solid line in FIG. 3(a). A temperature characteristics G of a luminous material for G color is represented by a concaved curve having minimum value at a center portion in the vicinity of 0° C. as shown by a solid line in FIG. 3(b) and a temperature characteristics B of a luminous material for B color is linearly increasing with increase of temperature as shown by a solid line in FIG. 3(c).
Incidentally, ordinate in FIG. 3(a) to FIG. 3(c) indicates ratio of luminance Po to output current Io, that is, luminance per unit drive current (Po/Io) and abscissa indicates temperature (° C.). Luminance Po of luminous material is usually linear with respect to the output current Io in the operable temperature range of the organic EL element and temperature characteristics of the R, G and B colors are different from each other.
As a result, white balance on the display screen of the display device, which is set at a center temperature, which is usually normal temperature such as 25° C., in the temperature range may be lost if the operable temperature of the display device is changed within the operable temperature range from −10° C. to +70° C.